System Bits: Oct. 8

The next big thing in particle accelerators
Stanford University engineers have helped create what may be the next big thing in particle accelerators – and it fits on a fingertip.

In a project that included scientists from the U.S. Department of Energy’s SLAC National Accelerator Laboratory, a linear accelerator two miles long, accelerators energized charged particles to accomplish a range of tasks from generating x-rays for medical imaging systems to unraveling the mysteries of matter.

Electromagnetic waves were used to boost the energy of particles although many conventional accelerators employ microwave radiation and bulky copper tubing to generate a wavy electric field. Particles surf the finely-tuned waves, hitting swell after swell, and accelerating to higher and higher energies.

The new device miniaturizes this process by using a series of nanoscopic ravines etched into the chip by researchers at the Stanford Nanofabrication Facility (SNF). On the accelerator chip, a laser shoots through the nanofabricated slits to create the waves that boost the particles’ energies. But unlike in large, conventional accelerators, the new device creates these waves in centimeters.

A computer simulation of the glass accelerator chip shows the accelerating (orange) and decelerating (blue) components of the electromagnetic waves. The glass slits are the tooth-like features on both sides of the central electron conduit. (Source: Stanford University)

In this first proof-of-principle demonstration, the researchers were already able to see particle accelerating gradients that are much higher than what can be achieved with conventional accelerators.

Fool’s gold holds promise
Holding promise as a high-tech material with potential uses in solar cells, spintronic devices and catalysts, pyrite — better known as ‘fool’s gold’ for its yellowish metallic appearance — is a common, naturally occurring mineral and is also a byproduct of corrosion of steel in deep-sea oil and gas wells. Its potential usefulness in devices and its role in corrosion are largely influenced by the fundamental electronic properties of its surface which have remained relatively unexplored until now as a team of MIT researches has developed a way to probe the surface properties.

The surface of this material is very different from the bulk, something that is common for many materials and while the bulk has been widely characterized, when it comes to the surface, there is only a small amount of data, and it’s not consistent.

The new work was made possible by combining scanning tunneling spectroscopy (STS), a tool developed in the 1980s, with modern computational methods to interpret its output, the researchers said. They needed a tool that measures to a shallow depth of one or two atomic layers on a sample, but also the computational ability to establish how the surface behaves differently from the bulk, and how to model the experimental data.

New measurements reveal that on pyrite’s surface, a property called an energy bandgap — essential for making solar cells or semiconductor devices — has a value less than half that of the bulk material. Previous studies had produced conflicting results for the bandgap at the surface. In order to uncover the true nature of this surface, the researchers performed tunneling spectroscopy measurements on pristine pyrite surfaces, and analyzed the results with theoretical modeling of the tunneling spectra. The model was adapted from semiconductor physics, and informed by electronic structure calculations.

The reason for the discrepancy between surface and bulk properties, is that the surface has dangling bonds that provide electronic states into the bulk bandgap.

Pyrite has significant potential in its own right, the researchers insisted. Its possible use as a material for solar cells was investigated as far back as the 1980s and while the new findings don’t solve pyrite’s low-bandgap problem on its surface, they do explain why it works so poorly, despite the expected high performance based on the bandgap of the bulk material. There could be ways to treat the surface to eliminate the problem, they said.

If so, pyrite could find significant use in solar cells, since the material itself is abundant and inexpensive. It has also been suggested as a possible material for spintronic devices, in which the spin of electrons carries information.